1. Field of the Invention
The present invention relates to an axial flow fan used as a fan for electronic devices, and more specifically, it relates to a structure of an axial flow fan suitable for high efficiency and low noise level.
2. Description of the Related Art
An axial flow fan is used for various kinds of appliances such as a fan for cooling electronic devices and an outdoor unit of air-conditioners, and a variety of technologies have been developed for realizing high efficiency and low noise level thereof.
As for a fan casing, there is a technology for reducing the noise level by forming a cylindrical inlet of the fan casing and forming an axisymmetric suction flow (for example, refer to Patent Document 1).
As for a fan shape, there is provided a technology of realizing high efficiency and low noise level by forming a triangular leading edge at a blade tip by advancing the edge in a rotational direction, tilting the blade toward an inlet side, or designing the camber and the setting angle to be in an adequate range to reduce tip vortexes and leak flow (for example, refer to Patent Documents 2 to 5).
There is further provided a technology of realizing low noise level by improving a shape of a blade tip (for example, refer to Patent Document 6).
There is still further provided a technology of realizing high efficiency by improving a shape of a trailing edge (for example, refer to Patent Document 7).
Patent Document 1
Japanese Unexamined Patent Application Publication No. 61-190198 (Pages 2 to 3, FIGS. 1 to 3)
Patent Document 2
Japanese Unexamined Patent Application Publication No. 61-065096 (Pages 5 to 6, FIGS. 1 and 2)
Patent Document 3
Japanese Unexamined Patent Application Publication No. 09-049500 (Pages 13 to 14, FIGS. 1 to 7)
Patent Document 4
Japanese Unexamined Patent Application Publication No. 11-044432 (Pages 4 to 6, FIGS. 1 to 7)
Patent Document 5
Japanese Unexamined Patent Application Publication No. 08-303391 (Page 2, FIGS. 1 to 5)
Patent Document 6
Japanese Unexamined Patent Application Publication No. 06-129397 (Page 3, FIGS. 1 to 3)
Patent Document 7
Japanese Unexamined Patent Application Publication No. 2002-257088 (Page 4, FIGS. 1 and 2) Non-Patent Document 1
“Turbo-fan and compressor” by NAMAI, Takefumi and INOUE, Masahiro Corona, Published on Aug. 25, 1988, pp357–418
Technical development of the axial flow fan has been advancing for a long time, and the axial flow fan has become a well-developed mechanical element. In the related art described above, sufficient effects have been achieved in realizing high efficiency and low noise level thereof.
However, these technologies have been focused on the versatility, and further improvement in performance has been difficult.
Most of the fans for cooling devices are mass-produced, in other words, catalog products, and it is difficult to specify service conditions and applications (Patent Documents 1 and 5).
Therefore, a design has been specified so that the sucked flow and the discharged flow are in the axial flow direction parallel to the rotation axis. More specifically, more work is done at a tip portion of a blade, in other words, at a blade tip. The pressure gradient is generated with the flow at the tip portion of the blade in a high pressure, the flow expanding outwardly by the centrifugal force of the rotation is suppressed, and allowed to flow in the axial flow direction.
Even in the axial flow fan used for air-conditioners, the flow is designed to flow in the axial flow direction similar to the above in order to avoid any circulation phenomenon that the discharged flow is sucked in again (Patent Documents 2 to 4, 6 and 7).
In a general structure of these axial flow fans, an adequate tip clearance is ensured between the tip and the fan casing. When the impeller is rotated, tip vortexes and leak flow occur in the tip clearance due to the pressure difference between the pressure surface and the suction surface of the blade and the pressure difference between the suction side and the discharge side, and cause losses and noise.
In addition, a boundary layer of the fan casing is twisted by the flow field between a stationary fan casing wall surface and the rotating impeller, the flow is interfered with tip vortexes, leak flow or the like at the tip clearance, and the flow becomes very complex.
However, the tangential velocity is largest, and more work is done at the tip portion. Therefore, most of the known axial flow fans have been designed with design scheme of doing more work by such complex flows at the tip portion.
As described above, more work means that the absolute value of losses is large even it is assumed that the ratio of the energy taken out of the input energy is unchanged. In other words, setting the flow in the axial direction and reduction of losses and noise at the tip portion are in a trade-off relationship, and a problem occurs when realizing higher efficiency and lower noise level.